Method and apparatus for measuring electrical conductivity



Aug.- 6, 1935. A. PREISMAN 2,010,243

METHOD AND APPARATUS FOR MEASURING ELECTRICAL CONDUCTIVITY Original Filed April 17, 1928 INVENTOR Patented Aug. 6, 1 935 Q r I UNITED STATES PATENT OFFICE METHOD AND APPARATUS FOR MEASURING EL ECTRICAL CONDUCTIVITY Albert Preisman, New York, N. Y.

- Original application April 17, 1928, Serial No; 270,741. Divided and this application Feb-.

ruary 24, 1932, Serial No. 594,879

10 Claims. (Cl. 175-183) My present invention relates to methods of, In the said original cop'ending application it and apparatus for electrical measurements, and has been demonstrated from purely mathematihas particular reference to a method of, and an- .cal and physical id r t o Sustained by paratus for ascertaining the electrical conducexperimental verification, that by measuring the 5 tivity of materials, the present application being plate current cra audion tube having the 5 a division of my copending application Serial material to be measured connected in series No. 270,741, filed April 1'7, 1928, now Patent No. between a grid battery and the grid, I can ascer- 1,966,185. tain quantitativelythe conductivity of the ma- Practical experience has revealed various deterial even if this be very low and comparable l fects in ohmmeters in present usage. For exwith'the leakage in the tube and associated l0 ample if resistances are measured which are p I e addit n thereto, devised eindicated near the infinity end of the scale of rious arrangements which enable this fundapresent day ohmmeters, wholly unreliable readmental invention to be readily operated with any ings are obtained. type of audion' tube, and applied to uses to be 5 Again ohmmeters now in use that are adapted he inafter fu y e dto measure high resistances are expensive and While one of the principal uses of ,the prinbuilt of dynamic parts that are delicate. Moreciples above described is for the measurement over, they cannot utilize alternating current as of resistance or conductivity, especially if the a source of potential. All these limitations handiformer be o!v exceedingly high values, as from one cap their every day use. to one thousand megohms, or more, the inven- 20 Now, I nave discovered certain principles intion is also applicable to other uses. One of volved in the performance of the well known these uses involves the employment of the inaudion or electronic discharge tube, and applied vention as a groundjdetector in an ungrounded them in an effort to produce an instrument which electrical system, suchas a two'phase four wire will not only be an ohmmeter capable of acungrounded system.

curately measuring resistances, but one that is Accordingly one of the main objects of my incapable of many independent uses, and yet be vention is to provide a method of, and apparatus comparatively inei'q'pensive, rugged in construcfor indicating quantitatively the conductivity of tion, and capable of use with alternating curmaterials, comprising inserting such materials in rent as well as direct current. series with the grid of an electronic discharge 30 Heretofore it has been considered that if the tube, and a source of grid potential, the value of d e t of an e e tron c disc arge t be, the plate current of said tube at a specific grid hereinafter referred to as an audion for brevity, potential being a measure of the conductivity of be insulated from the filament element by a the particular material being measured.

resistance of forty me o e, t n t Wi l Another. object of my invention is to provide 35 be Thai? is y it will assume a a direct reading ohmmeter comprising an elec- DOtentiel in most Cases Which is negative with tronic discharge'tube having a material to be pe t ii 't d negative end of the fi measured and a source of grid potential in series Again, Prior to my discovery, it has been with the grid of the tube, the said grid potential 40 thought that the free grid potentia e t a source being adjustable to render the ohmmeter 40 ieproducible Value but that it ,depends. upon multi-range, and independent of tube or voltage the values of the filament and plate voltage, variation and also upon which of the two circuits, the other objeqts of my invention are t improve Plate filament, is closed first generally the simplicity and efllciency of such de- 5 But, us S a d UX201A, UX240 and UXZIO vices and to provide a device or apparatus of this tubes, 1 have found the free grid to e lways kind which is durable, reliable in operation. and negative relative to the filament. Again, I have economical to manufacture v discovered that contrary to the aforementioned still other cbjects of my invention will appear teachings of the prior art, the potential of a as the description proceeds, and such objects fre grid, a measured by the p a e' should be considered as included in the herein 50 is always the same for the filament and plate statement of the objects of invention. potentials, or if both of these be increased or The invention may now be more fully underdecreased by a reasonably small amount, the stood from the following description when read in plate current due to the free grid is practiconnection with the accompanying drawing in rally unchanged which:

Fig. l is a circuit diagram of an apparatus, operated by direct current batteries, and adjustable for use with any audion tube, by means of which the method is carried out.

Fig. 2 is a circuit diagram showing the arrangement of the apparatus in Fig. 1, for connection to an alternating current source.

Fig. 3 shows an adaptation of the circuit of Fig.

2 as a ground detector for an ungrounded polyphase system.

Referring to Fig. 1, reference characters I, 2 and 3 designate the filament (electron emission electrode), grid and plate respectively of a tube of conventional design. The filament I is heated by a current source known as the filament or A Fig. 1, part of the B battery could be used as the grid battery. The material whose electrical characteristic is to be ascertained, is interposed in the circuit between the grid 2 and the battery I. In Fig. l the insulation resistance of a cable I5 is to be measured. It should be noted that the cable possesses capacity as well as resistance.

A potentiometer 8 is placed in the circuit between the unknown cable resistance I5 and the grid battery I in order to adjust the positivepotential being applied to the grid. This device is used to correct for tube or battery voltage variations. An insulated support 9 for two tap-offs I0, I I constitutes the adjusting mechanism. The support 9 operates as one unit and thereby, by means of switch I2 which allows either grid potential to be applied to the grid 2, gives the instrument two ranges since either tap II) or I I may be connected to the grid. The advantage of such an arrangement will be explained later.

A conventional two-point push button switch I3, or any other type of double-throw switch, connects the potentiometer to the unknown cable re sistance I5, or a calibrated high resistance III. The calibrated resistance is normally connected to the switch I3 and is in circuit between the grid potential source I and the grid 2. Its purpose is to enable the plate meter 4 to be referred to a predetermined mark by varying the grid potentiometer 8, thus ensuring a constant reference point regardless of battery or tube variations.

Guard terminals I9 and 2I of conventional design are interposed in the circuit from the insulation of cable I5 and the contacts 20 to the filament battery 5. The purpose of the guard terminals is to prevent false readings due to leakage, whether in the set, or over the surface of the material being tested. These expedients assure accurate determinations when dealing with extremely high resistances.

A pair of battery switches Ii and It control the filament battery and grid battery respectively, and in actual construction they may be operated a a unit in the ft .m of a two-pole single throw switch. To compensate for voltage variations in the battery 5 a filament rheostat I6 is inserted in the filament circuit. I

As previously explained the plate meter Ll measures the resis ance RX of the material between the contacts 20. If the resistance is infinite in value, as where the contacts 20 are not connected to anything, the grid 2 is essentially free. It assumes 'a negative charge with respect to filament I, thus causing the plate current to be very low. As this unknown resistance Rx decreases through finite values, the grid, with the aid of the grid battery I, is able to discharge to the cold end of the filament.

The grid thus becomes less negative thereby allowing the plate current Ip to increase. This increase is noted on the milliammeter 4 in the plate circuit which may be calibrated to read directly in megohms. As shown in Fig. 1, and it being understood that the switches I2, II and I8 are closed, the pointer is at the low end of the scale, the scale reading directly in values of resistance R. As the resistance of the material being measured increases, the pointer moves to the left. When resistance of infinite value is measured, as explained before, the pointer is near the extreme left or infinity position of the scale. This should be obvious from the fact that, as stated in the previous paragraph, when the. grid is "free (that is to say, the resistance between the contacts 20, 20 is infinite), the plate current is very low.

For every value of Rx there will be a corresponding value of'I provided all voltages are maintainedconstant at somepredetermined values. The adjustable external grid potential I insures this in the following manner. I have determined experimentally that in order to adapt different tubes of the same type to the same scale on the meter 4, it is necessary to adjust the zero-error correction thereon, to get the same free grid reading. This is done by letting the unknown resistance be infinity, as would be the case with air. Then the smallest Rx value is found that can be read on that scale range. In other words, it is necessary to adjust the instrument to read the same at the two extremes of the scale; namely, at Rrc== and R1=lowest value indicated on the scale.

This is accomplished by means of the calibrated resistance It and is based on the fact that while different tubes may vary in their actual plate currents for-a given value of Rx (unknown resistance) and EI; (total grid voltage) their RX vs. Ip curves are practically identical. Hence, if these curves for various tubes are matched at their extreme values, intermediate values will coincide to about an accuracy as can be detected on the meter I.

In use, then, assuming that it has been predetermined that when the potentiometer 8 has been tapped as shown in Fig. 1, the switch I2 closed on a desired contact, and the switches I'I, I8 closed, the meter t reads as shown in Fig. 1, when switch It is in normal position. That is to say, the resistance of thecalibrated element It is'being measured. Prior to this, the infinity resistance reading has been taken by leaving the contacts 20 free, and depressing the switch I3.

For different tubes this infinity reading on meter Il may vary, but by means of the zero-error correction on the meter the pointer may be set to the infinity mark on the scale. Then, if the known resistance is off the calibrated resistance mark on the scale, by shifting 9, and releasing switch I 3 so it is in. normal position, the pointer of the meter may be set over the calibrated value on the scale. Obviously, adjusting the grid voltage would have no ffect on the other end of the scale where the resistance is infinity.

Thus, when the cable I3 is now connected to the contacts 20, regardless of the fact that tubes have been changed or that a particular tube has been used for a long time, the switches II, I8 are closed and the pointer observed on the meter 4. If the pointer is not over the calibrated value of the scale, the potentiometer 8 is adjusted until the pointer is over the value. Of course, the switch I2 is closed on either tap, depending on whether the higher potential represented by tap I is to be used, or whether the lower potential represented by tap I l is to be used.

Then the key I 3 is depressed and the pointer reading on the meter observed. Should other readings be taken, the key 13 constantly indicates due to its normal position that the instrument is reading true. If the battery strengths change, or any other variation occurs, the instrument can be made to read the same for all values of Rx between infinity and the lowest value that can be read on the meter, by the means described above.

The advantages of using an adjustable external grid potential source may be summarized as follows:

' 1. It makes the instrument more sensitive to large values of Rx.

2. At the larger values of Rx, the scale is more open.

3. In conjunction. with the zero-error adjustment of the meter, it enables different tubes to be adapted to the same scale on the plate meter, thus making the instrument a measuring device rather than a mere translating or indicating device.

4. It places Rx in series with the tube and socket leakage instead of in parallel to it, so that Rx can be read more accurately at high values, and also renders the instrument independent of relatively large variations in leakage.

As stated heretofore, my invention with some changes can be applied to an alternating current source. In Fig. 2 is shown the tube l, 2, 3 and plate meter 4 in circuit with the cable l5 to be measured and the calibrated resistance l4 as described heretofore. A double-pole single throw switch 40 closes the circuit and allows current from alternating current source 4| to flow therethrough. A transformer primary 44 has an adjustable resistance 45 connected thereto which compensates for variations in tube characteristics and line voltage. The function is similar to that of the adjustable potentiometer in Fig. '1.

A. grid and plate secondary 42 tapped by' a switch 46 serves to feed current to the plate and grid. By means of the switch 46 different grid voltages for difierent ranges of the instrument are procured. A filament secondary 43 connects to the filament. and heats the filament. An inductance 41, of ten henries or more, is inserted in the grid circuit to balance the capacity in the unknown cable.

In this alternating current modification the rectifying properties of the grid or control element are used as well as its amplifying effect, and due to the former effect, pulsating direct current is applied to the unknown resistance.

Since plate current Ip, flows only during the half cycles when the plate is positive, it is only during these half cycles that the grid has any effect on Ip, and since the grid is also made positive through Rx at these times, it-can vary I depending upon the value of Rx. t

Since RX may have capacity 0x as well, the inductance coil 41 is used to correct the decreasing effect of C5 upon I thus rendering the readings independent of Cx. This seems to be due to the fact that the inductance maintains the grid current, and therefore the grid potential, even after the potential E1; has decreased to zero again. This is probably due to the fact that the inductance spreads the grid current pulses over a greater portion of the half cycle and thus prevents the charge in Cx making the grid negative and thereby distorting the plate current by making it more peaked whereby the average or direct current value of it as measured on the plate milliammeter 4 would be lower.

In the operation of this alternating current embodiment, to calibrate the tube all voltages may be varied by adjusting the rheostat 45 in the primary circuit 44. This adjustment is available in addition to the infinity, or free grid, adjustment which is adjusted by means of the zero-error adjustment on the plate meter.

To obtain several ranges as explained before different values of El. may be used. To do this, the high tension secondary winding 42 is tapped at suitable points and connected through a rotary switch 46 to the line terminal.

Of course, this modification can be used in conjunction with any means for maintaining constant voltage across the primary and associated resistor.

Another application of my invention is as a ground detector on an ungrounded alternating current system. As shown in Fig. 3, the circuit is set up for one phase of a two phase ungrounded system. An auto transformer 60 is connected across the two live cables 6| which lead from the ungrounded secondary of the station-bank transformer 63. For a direct current system a resistor may be used instead of an auto-transformer.

The auto-transformer 50 is tapped by a series of three taps 64 designed to move as a unit on a support 62. The lead sheaths of the'cables 6| are grounded, as is also the grid of the audion tube.

. It should be noted that normally the lower two taps 64 are slightly above the center point of the transformer 60, so that, if the capacity and insulation resistance of both cables are equal (or insulation resistance negligible), ground is half way in potential between the two live conductors, and the filament is therefore slightly positive to ground and therefore to grid.

A two-point switch 65 connects the grid 2 to the ground, or to the center-tap of two condensers 66 connected in series, which act as impedances. The condensers are so arranged that they tap off 60 at any points desired. The voltage to ground from either live cable depends upon the relative values of Cx and Rx for the insulation of each cable.

If these are the same for the two, and are uniformly distributed on all pieces of connected apparatus, the voltage from each live cable to ground is one half of the voltage between the two cables.

The grid is at ground potential, and, in the above case, would be half-way in potential from either conductor. Suppose the plate current I may vary from zero to some safe allowable upper value, Ipm, and that the filament is adjusted on the auto-transformer to be at such a potential with respect to the grid that Ip: /ZIpm. If, on the other hand, the grid becomes less negative, or even positive, with respect to the filament, Ip increases above its normal value of Ipm,

Normally, Cx for the two cables remains constant if no changes are made in the circuit, and

.ascertaining the anode current or only Rx may vary as the cable ages. If the top cable grounds, Rx for its insulation decreases to a value, let us say, of ten megohms or less, while Rx for the bottom cable remains unchanged. The ground shifts toward the top cable in potential, as does the grid connected to it, while the filament connected to 6|, is kept at the same potential with respect to either cable as before. The grid thus becomes less negative with respect to the filament than it was before, or even positive to it, depending upon how effectively the top cable is grounded, and so Ip increases.

' If, on the other hand, the bottom cable grounds, the grid becomes more negative with respect to the filament, and Ip decreases. If both cables ground equally, Ip remains unchanged, but this is a fault common to practically all ground detectors, and is, moreover, unlikely to occur.

Ip would change if another tube were used, or if the characteristics of this tube were to change. To check this, the impcdances 66 are used. These may be resistances,inductances, or condensers. They are here shown as condensers, in order more closely to simulate the insulation impedance of the cables. These are designed to maintain their characteristics (impedance values) during the life of the detector, and when the latter is adjusted to the cable, the voltage across these two impedances is adjusted until, when the grid is switched over to their mid-point, the same plate current obtains. Hereafter, if I changes in normal operation, and does not when the grid is connected to their center tap, we know that the two conductors have changed in potential to ground, the change in I indicating which. cable is at fault.

It will be obvious that the general principles herein disclosed may be employed in many other organizations widely different from those illustrated, without departing from the spirit of the invention as defined in the following claims.

What I claim is:

1. An apparatus of the class described, adapted to detect grounds on an ungrounded current conducting system, comprising audion means adapted to be connected to conductors in said system, audion plate-current indicating means in circuit with said audion, and means associated with said audion to indicate which conductor is grounded.

2. A device of the class described adapted to ascertain the conductivity of live cables in an ungrounded system comprising an audion tube, the

grid of which is grounded, and connected to said cables, the insulation of each cable being connected between the conductor of each cable and the grid, means asssociatcd with said cable conductors to adiustably supply voltage to the filament and plate circuits of said tube, a plate meter in said plate circuit, variations in the leakage of any of said cables affecting the potential of said grid with respect to the filament whereby said plate meter indicates said variations, and means associated with said adjustable supply means to calibrate the device to render it independent of variations other than said insulation resistance variations.

3. A device comprising an electronic discharge tube having, at least, an electronic discharge cathode, anode, and control electrode, an ungrounded electrical system having conductors;

-means for obtaining a point of potential whose magnitude is fixed with respect to any desired two of the conductors; means for connecting the cathode to this point, means for grounding the control electrode, means in the anode circuit for its effects,

whereby changes in the anode current or its effectsshow by their magnitude the amount of potential unbalance to ground of the two conductors, and by the direction of the change which conductor has changed its potential to ground from its original value.

4. A device comprising an electronic discharge tube having, at least, an electronic discharge cathode, anode, and control electrode, an ungrounded electrical system having conductors, means for obtaining a point of potential whose magnitude is fixed with respect to any desired two of the conductors, means for connecting the cathode to this point, meansfor connecting the anode to a point at some potential at least periodically positive with respect to the cathode, means in the anode circuit for ascertaining the anode current or its effects, means adapted to vary the indication of said measuring means for the purposes of calibration, two impedances connected in series across said means for obtaining a point of potential whose magnitude is fixed with respect to any desired two of the conductors, and means for associating either the center point between the above impedances, or ground, with the control electrode.

5. A device comprising an electronic discharge tube having, at least, an electronic discharge cathode, anode, and control electrode, an ungrounded electrical system having conductors, potentiometric means for obtaining a point of potential whose magnitude is fixed with respect to any two of said conductors across which potentiometric means is connected, means for connecting the cathode to this point, means for connecting the anode to a point on said potentiometric means which is at least periodically positive with respect to the cathode, means in the anode circuit for as-' certaining the anode current or its effects, means adapted to vary the indication of said measuring means for the purposes of calibration, two impedances connected in series across a suitable portion of said potentiometric means and means for associating either the center point between the above impedances, or ground, with the control electrode.

6. A method of ground detection of an ungrounded electrical system, which consists in employing a system having a control and controlled circuits, maintaining one point in the control system at ground potential, and another point at a predetermined potential with respect to any desired conductor of the ungrounded system, and using the electrostatic potential difference between the aforesaid points as a means of directly controlling'and thereby indicating effects in the controlled circuit, said efiects serving as an indication of the electrostatic potential to groundof said conductor.

7. A method of detecting an electrostatic difference of potential between two charged bodies by maintaining the electronic discharge cathode of an electronic discharge tube at a fixed potential with respect to the two bodies, and maintaining the control electrode at the potential of some fixed reference point, such as ground, and noting variations, in the potential between these two electrodes, in the anode circuit.

8. A device comprising an electronic discharge tube provided with at least a cathode, anode and a control electrode, two conductors electrically independent of each other, means for obtaining a point of potential whose magnitude is fixed with respect to one of the conductors, means for associating the said cathode with this point, means for associating said control electrode with the other conductor, and means in the anode circuit of said tube to indicate the electrostatic potential between the above two conductors.

9. An apparatus, adapted to detect grounds on an ungrounded current conducting system including at least two conductors, comprising a tube having its cathode maintained at a fixed potential with respect to said conductors, the control grid of said tube being grounded, and means in the anode circuit of the tube for indicating the po-- tential difference between said grid and cathode. 10. An arrangement for detecting grounding of an ungrounded electrical system including at least two conductors comprising a tube provided with a grounded grid, means for establishing the cath- ALBERT PREISMAN. 

